The invention relates generally to gyros and inertial sensing systems, and more particularly to free rotor gyros with means applying torque to reposition the rotor through circumferentially distributed torquers.
A free rotor gyro requires a spinning rotor suspended in a case which can be turned or moved without affecting the angular orientation of the rotor axis (called "spin axis") with respect to inertial space.
In one type of free rotor gyro, a rotor in the form of an outer inertial ring is pivotally connected to an inner gimbal ring which in turn is orthogonally pivotally connected to a central drive shaft. The axis of rotation of the drive shaft is fixed relative to the case. When used as a "strap-down" gyro for vehicular attitude reference, the case (and thus the drive axis) are fixed to a vehicle such as a space craft; the rotor is free relative to the vehicle but fixed relative to inertial space. The rotor gimbal ring and drive shaft are initially coaxial and the rotor and gimbal may be thought of as occupying the same nominal plane. When the case is reoriented to another position, the drive axis makes an angle with the spin axis. Because of the orthogonal pivots, the plane of the gimbal ring alternates between the new plane of the rotor and the plane normal to the drive axis twice per revolution. The accelerating torques which continually reorient the gimbal ring are transmitted to the rotor as small "wobble" torques, proportional to the rotor angle and the square of the spin rate.
This relationship is used to advantage in a "dynamically tuned" free rotor gyro which employs torsional flexures for the pivots. The spring torques of the flexures proportionally resist deflection and are exactly matched at a uniquely determined spin rate by the antispring effect of the wobble torques. Dynamic cancellation decouples the rotor spin axis from the case.
In the strap-down gyro, discrete circumferentially distributed torquers are used to constantly reposition the rotor to the normal plane whenever the case turns. Normally there are four D'Arsonval coil type or solenoid type torquers beneath the rotor paired on orthogonal axes. Corresponding pickoffs are also mounted to the case beneath the rotor to sense deflection. The torquer current, controlled by a rebalance servo network responsive to the pickoffs, is sensed and processed to determine angular rate about any two mutually orthogonal input axes that are located in a plane perpendicular to the spin axis.
The positioning of the four pickoffs relative to the four torquers is critical. With D'Arsonval type torquer coils, the pickoffs are typically centered with respect to the segmental arc of a corresponding coil. However, with solenoid torquers, the axes on which the pickoffs are arranged are typically shifted 45.degree. relative to the torquer axes to make use of the extra space. In either case, it is difficult to have the pickoffs diametrically opposite from the torquers at the same radial distance.